The unexpected role that a unit of white blood cells can play in elucidating tumor targets
The immune system is a vast army of skilled assassins that swoop in for the kill. Some of the earliest responders are polymorphonuclear neutrophils (PMNs), white blood cells containing granules that help break down the membranes of target cells. Because PMNs are so abundant, they are one of the most powerful defenses against infections. Under certain circumstances, they also can have a powerful effect against tumors. The question is how?
For years, researchers assumed these innate cells engulfed, digested and ultimately destroyed their pathogen prey using a complex system known as phagocytosis. While this is true, PMNs can also provoke target cells to chop up and hand over parts of their cell membrane. This process, known as trogocytosis, from the Greek word to gnaw, has been a hot topic ever since a 2017 study published in the journal Blood found that it appeared to maximize the chances that therapeutic antibodies find their tumor targets.
Therapeutic antibodies work in two different ways: either by attaching to and blocking growth-promoting soluble ligands or by binding to receptors on the surface of cancer cells (or other nearby cells) that help cancer cells grow or spread. The second group usually activates the innate immune defense by Fc-mediated activation of natural killer cells, macrophages, or the complement system or the specific immune defense by activation of T-cells, for instance. It can even be a mixture of both.
The Blood study compared two targeted antibodies for chronic lymphocytic leukemia— rituximab and obinutuzumab—and found evidence of trogocytosis but not phagocytosis in how PMNs alert antibodies to their tumor targets, in this case by drawing attention to “flags” called antigens that are presented on the surface of cells. The study used a variety of techniques, including live-cell time-lapse microscopy, confocal microscopy, and flow cytometry, to demonstrate this.
While there is still much more to explore about the precise role of trogocytosis, it has had implications on future drug development including biosimilarity assessment for follow-on biologics. In the presence of therapeutic antibodies, it became important to know if trogocytosis occurred and allows those PMNs abundant in all of us to act as additional antigen-presenting cells. Having this information in hand is another step towards shedding light on the complex mechanisms of action of therapeutic antibodies in the patient.
Toward that end, our lab developed a primary cell-based assay to evaluate that functional capacity, though it wasn’t easy. Granulocytes only live about a day so you have to move fast in isolating them. Then you need to label the target cells with fluorescent dye. If antibodies are present and the isolated granulocyte are in the right concentration and right place, you can show this effect of transferring membrane fragments from target cells to the cell surface e.g. by flow cytometry or by fluorescence microscopy—a louder siren for antibodies seeking their targets.